Antimalarial activity of pyrroloiminoquinones from the Australian marine sponge Zyzzya sp

A new bispyrroloiminoquinone alkaloid, tsitsikammamine C (1), displayed potent in vitro antimalarial activity with IC(50) values of 13 and 18 nM against chloroquine-sensitive (3D7) and chloroquine-resistant (Dd2) Plasmodium falciparum, respectively. Tsitsikammamine C (1) displayed selectivity indices of >200 against HEK293 cells and inhibited both ring and trophozoite stages of the malaria parasite life cycle. Previously reported compounds makaluvamines J (2), G (3), L (4), K (5) and damirones A (6) and B (7) were also isolated from the same marine sponge (Zyzzya sp.). Compounds 2-4 displayed potent growth inhibitory activity (IC(50) < 100 nM) against both P. falciparum lines and only moderate cytotoxicity against HEK293 cells (IC(50) = 1-4 μM). Makaluvamine G (3) was not toxic to mice and suppressed parasite growth in P. berghei infected mice following subcutaneous administration at 8 mg kg(-1) day(-1).     

Synthesis and antimalarial activity of 2-aziridinyl- and 2,3-bis(aziridinyl)-1,4-naphthoquinonyl sulfonate and acylate derivatives

A series of 2-aziridinyl- and 2,3-bis(aziridinyl)-1,4-naphthoquinonyl sulfonate and acylate derivatives has been synthesized and evaluated for antimalarial activity in vitro against the human malaria parasite, Plasmodium falciparum (Vietnam Smith strain, chloroquine-resistant at the R3 level). The most active compounds, 2-aziridinyl-1,4-naphthoquinon-5-yl p-ethylbenzenesulfonate (13), 2-aziridinyl-1,4-naphthoquinon-5-yl p-tert-butylbenzenesulfonate (48), and 2-aziridinyl-5-hydroxy-1,4-naphthoquinone (5) produced 50% inhibition of the growth of P. falciparum at 9.6 x 10(-8), 2.4 x 10(-8), and 8.8 x 10(-8) M, respectively.     

Synthesis and evaluation of new antimalarial phenylurenyl chalcone derivatives

Phenylurenyl chalcone derivatives have been synthesized and tested as inhibitors of in vitro development of a chloroquine-resistant strain of Plasmodium falciparum, activity of the cysteine protease falcipain-2, in vitro globin hydrolysis, beta-hematin formation, and murine Plasmodium berghei malaria. The most active antimalarial compound was 1-[3'-N-(N'-phenylurenyl)phenyl]-3(3,4,5-trimethoxyphenyl)-2-propen-1-one 49, with an IC(50) of 1.76 microM for inhibition of P. falciparum development. Results suggest that chalcones exert their antimalarial activity via multiple mechanisms.     

Antiplasmodial phenolic compounds from Piptadenia pervillei

Piptadenia pervillei Vatke (Fabaceae) was selected from a screening programme devoted to the search of naturally-occuring antimalarial compounds from plants of Madagascar. Bioassay-guided fractionation of the ethyl acetate extract of the leaves led to the isolation of four phenolic compounds, (+)-catechin ( 1), (+)-catechin 5-gallate ( 2), (+)-catechin 3-gallate ( 3) and ethyl gallate ( 4). Structures were determined by NMR and mass spectroscopy. Compounds 2 and 3 displayed the highest in vitro activity against the chloroquine-resistant strain FcB1 of Plasmodium falciparum with IC (50) values of 1.2 microM and 1.0 microM, respectively, and no significant cytotoxicity against the human embryonic lung cells MRC-5 was measured (IC (50) values > 75 microM). Five analogues ( 5 - 9) of (+)-catechin 5-gallate ( 2) were synthesized and evaluated for their antiplasmodial activity.     

In vitro and in vivo antimalarial properties of isostrychnopentamine, an indolomonoterpenic alkaloid from Strychnos usambarensis

Isostrychnopentamine (ISP) is an asymmetric indolomonoterpenic alkaloid isolated from the leaves of Strychnos usambarensis. The in vitro antiplasmodial activities against five Plasmodium falciparum cell lines were evaluated: ISP possessed an in vitro IC (50) near 0.1 microM against all P. falciparum cell lines tested (chloroquine-resistant and chloroquine-sensitive lines) and showed antiplasmodial selectivity compared to cytotoxicity on human cell lines. The stage-dependent susceptibility to a short exposure of ISP was then investigated. The ring stage was shown to be the most sensitive one, but all stages were affected by ISP treatment. By means of fluorescence microscopy, it was shown that ISP was not accumulated inside the food vacuole of the parasite. Finally, the in vivo antimalarial activities against the P. berghei NK173 and P. vinckei petteri murine strains were determined. The ED (50) in vivo was about 30 mg/kg/day by the intraperitoneal route (after 4 days treatment).     

Identification of phosphinate dipeptide analog inhibitors directed against the Plasmodium falciparum M17 leucine aminopeptidase as lead antimalarial compounds

Previous studies have pinpointed the M17 leucyl aminopeptidase of Plasmodium falciparum (PfLAP) as a target for the development of new antimalarials. This metallo-exopeptidase functions in the terminal stages of hemoglobin digestion and is inhibited by bestatin, a natural analog of Phe-Leu. By screening novel phosphinate dipeptide analogues for inhibitory activity against recombinant PfLAP, we have discovered two compounds, 4 (hPheP[CH2]Phe) and 5 (hPheP[CH2]Tyr), with inhibitory constants better than bestatin. These compounds are fast, tight-binding inhibitors that make improved contacts within the active site of PfLAP. Both compounds inhibit the growth of P. falciparum in vitro, exhibiting IC50 values against the chloroquine-resistant clone Dd2 of 20-40 and 12-23 muM, respectively. While bestatin exhibited some in vivo activity against Plasmodium chabaudi chabaudi, compound 4 reduced parasite burden by 92%. These studies establish the PfLAP as a prime target for the development of antimalarial drugs and provide important new lead compounds.     

Antiplasmodial activity of Uvaria klaineana

Crude extracts of Uvaria klaineana Engler and Diels (Annonaceae) stems showed in vitro activity against chloroquine-resistant K1 strain of Plasmodium falciparum. The most active extract was the basic dichloromethane extract containing crude alkaloids (IC50 = 3.55 microg/mL). The bioassay-guided fractionation of this extract led to the isolation of the major alkaloid crotsparine (1) which showed an antiplasmodial activity against the chloroquine-sensitive Thai strain of P. falciparum and the chloroquine-resistant K1 and FcB1 strains of P. falciparum. Two minor alkaloids were also identified as crotonosine (2) and zenkerine (3). Their structures were elucidated using 2D-NMR techniques.     

Antiplasmodial and cytotoxic activity of galipinine and other tetrahydroquinolines from Galipea officinalis

The antimalarial and toxicological properties of four tetrahydroquinoline alkaloids from Galipea officinalis trunk bark were studied. Crude extracts and pure alkaloids were tested for in vitro antimalarial activity on Plasmodium falciparum. The IC50 were evaluated after 24 and 72 h contact between compounds and the parasite culture, and ranged from 1.8 to 40 microg/ml for the chloroquine-sensitive strain (CQS) and from 0.09 to 38 microg/ml for the chloroquine-resistant strains (CQR). Galipinine yielded the best antimalarial effect (IC50: 0.09 - 0.9 microg/ml on CQR strain) and this compound interacted particularly between the 32(nd) and the 40(th) hour of the P. falciparum erythrocytic cycle. The cytotoxicity of the extracts and pure tetrahydroquinoline alkaloids was assessed on the HeLa cell line and showed IC50 values ranging from 5.8 to above 50 microg/ml.     

Antimalarial polyamine analogues

A series of novel tetraamines of the general formula RNH(CH2)xNH(CH2)yNH(CH2)xNHR was synthesized and examined for activity against growth of Plasmodium falciparum in vitro. Within the series, dibenzyl analogues (R = benzyl) were found to be the most effective growth inhibitors, with IC50 values of about 10(-6) M. Further modifications of the tetraamine provided the optimum chain length for antimalarial activity of y = 7, x = 3. Compound 8 (MDL 27,695) with the structure y = 7, x = 3, R = benzyl, in combination with the ornithine decarboxylase inhibitor alpha-(difluoromethyl)ornithine, resulted in radical cures when tested against experimental Plasmodium berghei infections in mice. The structure-activity relationships of the series are discussed.     

Antimalarial agents. 1. alpha-Santonin-derived cyclic peroxide as potential antimalarial agent

An alpha-santonin-derived cyclic peroxide (7) related to qinghaosu (1) has been synthesized and tested for antimalarial activity in vitro against the chloroquine-resistant (Smith) isolates of Plasmodium falciparum as well as in vivo against Plasmodium berghei in mice and was found to be devoid of activity.     

Synthesis, cytotoxicity and antimalarial activity of ferrocenyl amides of 4-aminoquinolines

Series of 4-aminoquinolines bearing an amino side chain linked to the ferrocene moiety through an amide bond were synthesized and evaluated for their antimalarial activity against both chloroquine-sensitive (D10, CQ-S) and chloroquine-resistant (Dd2, CQ-R) strains of Plasmodium falciparum. They were also tested for cytotoxicity against Chinese Hamster Ovarian (CHO) cells. Amide 12 featuring propyl side chain linked to the ferrocene ring was the most active of all tested compounds. With an IC50 value of 0.08 microg/mL, this amide showed 1.5-fold higher activity than chloroquine diphosphate (IC50 = 0.12 microg/mL) against the resistant strain, with a selectivity index of 550 indicating its high selectivity towards the parasite. Derivatives which were equipotent against both strains also showed up to ten-fold increase in activity compared to primaquine.     

A short synthesis and biological evaluation of potent and nontoxic antimalarial bridged bicyclic beta-sulfonyl-endoperoxides

The syntheses and in vitro antimalarial screening of 50 bridged, bicyclic endoperoxides of types 9-13 are reported. In contrast to antimalarial trioxanes of the artemisinin family, but like yingzhaosu A and arteflene, the peroxide function of compounds 9-13 is contained in a 2,3-dioxabicyclo[3.3.1]nonane system 6. Peroxides 9 and 10 (R(1) = OH) are readily available through a multicomponent, sequential, free-radical reaction involving thiol-monoterpenes co-oxygenation (a TOCO reaction). beta-Sulfenyl peroxides 9 and 10 (R(1) = OH) are converted into beta-sulfinyl and beta-sulfonyl peroxides of types 11-13 by controlled S-oxidation and manipulation of the tert-hydroxyl group through acylation, alkylation, or dehydration followed by selective hydrogenation. Ten enantiopure beta-sulfonyl peroxides of types 12 and 13 exhibit in vitro antimalarial activity comparable to that of artemisinin (IC(50) = 6-24 nM against Plasmodium falciparum NF54). In vivo testing of a few selected peroxides against Plasmodium berghei N indicates that the antimalarial efficacies of beta-sulfonyl peroxides 39a, 46a, 46b, and 50a are comparable to those of some of the best antimalarial drugs and are higher than artemisinin against chloroquine-resistant Plasmodium yoelii ssp. NS. In view of the nontoxicity of beta-sulfonyl peroxides 39a, 46a, and 46b in mice, at high dosing, these compounds are regarded as promising antimalarial drug candidates.     

Bisquinolines. 1. N,N-bis(7-chloroquinolin-4-yl)alkanediamines with potential against chloroquine-resistant malaria.

On the basis of observations that several bisquinolines such as piperaquine possess notable activity against chloroquine-resistant malaria, 13 N,N-bis-(7-chloroquinolin-4-yl)alkanediamines were synthesized and screened against Plasmodium falciparum in vitro and Plasmodium berghei in vivo. Twelve of the thirteen bisquinolines had a significantly lower resistance index than did chloroquine; the resistance index was apparently unrelated to either in vitro or in vivo activity. Except for two compounds, there was a reasonable correlation between in vitro and in vivo activities. Seven of the thirteen bisquinolines had IC50's of less than 6 nM against both chloroquine-sensitive (D-6) and -resistant (W-2) clones of P. falciparum and were curative against P. berghei at doses of 640 mg/kg. In contrast to chloroquine, these bisquinolines did not show any toxic deaths at curative dose levels. Four bisquinolines, however, caused skin lesions at the site of injection. Maximum activity was seen in bisquinolines with a connecting bridge of two carbon atoms where decreased conformational mobility seemed to increase activity. Bisquinoline 3 (+/-)-trans-N1,N2-bis(7-chloroquinolin-4-yl)cyclohexane-1,2-diamin e was not only the most potent bisquinoline in vitro, but was clearly unique in its in vivo activity--80% and 100% cure rates were achieved at doses of 160 and 320 mg/kg, respectively. In summary, these preliminary results support the premise that bisquinolines may be useful agents against chloroquine-resistant malaria.     

Dispiro-1,2,4,5-tetraoxanes: a new class of antimalarial peroxides.

Dispiro-1,2,4,5-tetraoxanes 2-4 were synthesized as potential peroxide antimalarial drugs. They had curative activity against Plasmodium berghei in vivo at single doses of 320 and 640 mg/kg which confirms earlier unpublished data. Moreover, artemisinin (1) and 4 had equivalent ED50's against P. berghei in vivo in the multiple-dose Thompson test; neither showed any evidence of acute toxicity at total doses of more than 12 g/kg. Dispiro-1,2,4,5-tetraoxane 4 had IC50's comparable to those of 1 against Plasmodium falciparum clones in vitro. These results confirm the potential of dispiro-1,2,4,5-tetraoxanes as a new class of inexpensive peroxide antimalarial drugs.     

Synthesis and in vitro studies of novel pyrimidinyl peptidomimetics as potential antimalarial therapeutic agents

A class of new pyrimidinyl peptidomimetic agents (compounds 1-6) were synthesized, and their in vitro antimalarial activities against Plasmodium falciparum were evaluated. The core structure of the new agents consists of a substituted 5-aminopyrimidone ring and a Michael acceptor side chain methyl 2-hydroxymethyl-but-2-enoate. The synthesis of 1-6 featured a Baylis-Hillman reaction of various aldehydes with methyl acrylate catalyzed by 1,4-diazabicyclo[2.2.2]octane (DABCO) and a S(N)2' Mitsunobu reaction under the conditions of diethyl azadicarboxylate (DEAD), triphenylphosphine (Ph(3)P), and various acids. The new compounds exhibited potent in vitro growth inhibitory activity (IC (50) = 10-30 ng/mL) against both chloroquine sensitive (D-6) and chloroquine resistant (W-2) Plasmodium falciparum clones. Compound 6 (IC(50) = 6-8 ng/mL) is the most active compound of the class, the antimalarial efficacy of which is comparable to that of chloroquine. In general, this class of compound exhibited weak to moderate in vitro cytotoxicity against neuronal and macrophage cells with IC (50) in the range of 1-16 microg/mL and showed less toxicity in a colon cell line. Preliminary results indicated that compounds 3 and 6 are active against P. berghei, prolonged the life span of parasite-bearing mice from 6 days for untreated control to 16-24 days for drug-treated animals.     

Identification, design and biological evaluation of heterocyclic quinolones targeting Plasmodium falciparum type II NADH:quinone oxidoreductase (PfNDH2)

Following a program undertaken to identify hit compounds against NADH:ubiquinone oxidoreductase (PfNDH2), a novel enzyme target within the malaria parasite Plasmodium falciparum, hit to lead optimization led to identification of CK-2-68, a molecule suitable for further development. In order to reduce ClogP and improve solubility of CK-2-68 incorporation of a variety of heterocycles, within the side chain of the quinolone core, was carried out, and this approach led to a lead compound SL-2-25 (8b). 8b has IC(50)s in the nanomolar range versus both the enzyme and whole cell P. falciparum (IC(50) = 15 nM PfNDH2; IC(50) = 54 nM (3D7 strain of P. falciparum) with notable oral activity of ED(50)/ED(90) of 1.87/4.72 mg/kg versus Plasmodium berghei (NS Strain) in a murine model of malaria when formulated as a phosphate salt. Analogues in this series also demonstrate nanomolar activity against the bc(1) complex of P. falciparum providing the potential added benefit of a dual mechanism of action. The potent oral activity of 2-pyridyl quinolones underlines the potential of this template for further lead optimization studies.     

Enhanced activity of mefloquine and artesunic acid against Plasmodium falciparum in vitro and P. berghei in mice by combination with ciprofloxacin

The antimalarial activity of combinations of mefloquine or artesunic acid with ciprofloxacin and other synthetic fluoroquinolone was tested in vitro against Plasmodium falciparum using a strain (BHz26/86) partially resistant to chloroquine and a resistant clone (W2); both are sensitive to mefloquine. Inhibition of parasite growth was measured in relation to controls without drugs, either by counting parasitemia in Giemsa-stained blood smears or by measuring the reduction in [(3)H]-hypoxanthine uptake. Combinations containing artesunic acid or mefloquine with ciprofloxacin had significant in vitro activity, inhibiting by more than 90% of the growth of both strains of P. falciparum at doses significantly lower than those of the antimalarials alone. When tested in mice inoculated with P. berghei chloroquine-sensitive parasites (NK65 strain), ciprofloxacin was inactive, whereas mefloquine and artesunic acid were active (IC(50)=2.5 and 4.2 mg/kg, respectively); combinations containing mefloquine at an equivalent dose of 0.5 mg/kg reduced parasitemia by 59% and artesunic acid activity was also improved by ciprofloxacin. Our data support the idea that ciprofloxacin in combination with antimalarials may be useful in the treatment of chloroquine-resistant human malaria, allowing the use of lower doses of these drugs.     

Microwave-assisted ring opening of epoxides: a general route to the synthesis of 1-aminopropan-2-ols with anti malaria parasite activities

A series of 1-aminopropan-2-ols were synthesized and evaluated against two strains of malaria, Plasmodium falciparum FCR3 (chloroquine-resistant) and 3D7 (chloroquine-sensitive). Microwave-assisted ring opening of epoxides (aryl and alkyl glycidyl ethers, glycidol, epichlorohydrin) with various amines without catalysts generated the desired library of beta-amino alcohols rapidly and efficiently. Most of the compounds showed micromolar potency against malaria, with seven of them having IC50 values between 1 and 10 microM against both Plasmodium falciparum strains.     

Endoperoxides as potential antimalarial agents

A number of mono- and bicyclic endoperoxides were prepared and tested for antimalarial activity in search of a simplified analogue of the 5-oxygen-substituted 1,2,4-trioxane ring structure of the naturally occurring antimalarial qinghaosu. The compounds were assayed in an in vitro system for antimalarial activity against chloroquine-susceptible and chloroquine-resistant strains of P. falciparum. The most active compound in this assay was 2-[((butyloxy)-carbonyl)oxy]-1,1,10-trimethyl-6,9-epidioxy-delta 7-octalin (17a), which showed an IC50 of 100 and 57 ng/mL, respectively. For comparison, qinghaosu exhibits a mean IC50 less than 3.4 ng/mL.     

Antimalarial activity of lavandulyl flavanones isolated from the roots of Sophora flavescens

Four lavandulyl flavanones, (2S)-2'-methoxykurarinone (1), sophoraflavanone G (2), leachianone A (3), and (-)-kurarinone (4), which are isolated from the roots of Sophora flavescens have been tested for in vitro antimalarial activity against Plasmodium falciparum. Compounds 1-3 showed moderate antimalarial activities with EC(50) values of 2.4 x 10(-6), 2.6 x 10(-6), and 2.1 x 10(-6) M, respectively. These compounds did not show selective toxicity against P. falciparum in the toxicity test on mouse mammalian tumor cells, however, it is suggested that the position of methoxyl groups in flavanone skeleton plays an important role on antimalarial activity.